In a solar cell that is formed by an electrode and a P-I-N junction type amorphous silicon layer on a substrate, because the electromotive force of the silicon layer during operation is only 0.4 to 0.5 volts, when using the cell as a power source for, for example, an electronic wristwatch, it is not possible to charge a nickel-cadmium battery, i.e., secondary cells with this single solar cell.
Essentially, a single solar cell does not provide sufficient voltage to operate the necessary circuitry.
Because of the above-noted situation, an approach that is taken is that of placing a plurality of (for example, four) solar cells on a substrate, and connecting these solar cells in series so that the voltages thereof are added.
FIG. 2(F) shows a schematic cross-sectional representation of the structure of a solar cell of the past.
In this solar cell, a transparent conductive film 2 is formed on a glass substrate 1, and onto this a P-I-N junction type amorphous silicon film (hereinafter abbreviated as a-Si film) 3 is formed, over which a metallic. electrode 4 is further formed.
By the action of light that is incident to the substrate 1 from the bottom thereof, a photo-electromotive force that develops on the a-Si film can be extracted from the transparent conductive film 2 and the metallic electrode 4, which surround the a-Si film.
The structure of the laminate on the substrate as shown in FIG. 2(F), is divided to the left and right portions at approximately the center thereof, so that separate solar cells regions are formed on either side of the division.
As a convenience, the left and right solar cells will be referred to as elements A and B, respectively. The metallic electrode 4 of the element A is linked to the transparent conductive film 2 of the element B, thereby forming a series connection between the two elements.
Even in locations that are outside the range of FIG. 2(F), there are electrical connections between the metallic film 4 and the transparent conductive film 2 of neighboring elements, thereby forming a series connection between a plurality of solar cells that are formed on the substrate 1, so as to form a solar cell of the desired electromotive force.
A method for manufacturing a solar cell having the above-noted construction is briefly described below, with reference being made to FIG. 2.
As shown in FIG. 2(A), a transparent conductive film 2 is formed on the substrate 1 as an SiO.sub.2 film, using a CVD process such as thermal CVD. In order to etch this transparent conductive film 2, a photoresist film 7 is formed thereon, using photolithography.
Next, using the photoresist film 7 as a mask, the transparent conductive film 2 is etched, so as to remove the parts of the transparent conductive film 2 that are not masked by the photoresist film 7, thereby patterning the transparent conductive film 2.
After this is done, the photoresist film 7 is peeled away, thereby forming the transparent conductive film 2 into the electrodes having a desired pattern, as shown in FIG. 2(B).
Then, as shown in FIG. 2(C), a CVD process is used to laminate an a-Si film 3 over the transparent conductive film 2, and a photoresist film 8 is formed thereover using photolithography, for the purpose of etching the a-Si film 3.
Next, as shown in FIG. 2(D), using the photoresist film 8 as a mask, etching is performed once again, so as to pattern the a-Si film 3 into a desired pattern, and the photoresist film 8 is then peeled away, so as to form the a-Si film 3 into the desired shape.
Additionally, as shown in FIG. 2(E), a metallic film that will serve as the metallic electrode 4 is grown by sputtering over the entire surface of the transparent conductive film 2 and the a-Si film 3, and a photoresist film 9 for the purpose of etching them is formed thereon, using a third photolithography process.
Finally, as shown in FIG. 2(F), the photoresist film 9 is used as a mask to perform a third etching process, so as to pattern the metallic electrode 4, and the photoresist film is then peeled away.
The above process steps achieve the desired shape on the multi-layered portion formed on the substrate, with each region formed as a solar cell, an electrical connection being made between the metallic film 4 of the element A and the transparent conductive film 2 of the element B, thereby forming a series connection between elements A and B.
In another example of the past, as shown in FIG. 3(A), simultaneously with the lamination of the a-Si film 3 onto the transparent conductive film 2 with CVD method as shown in FIG. 2(C), the metallic electrode 4 is laminated, by performing an additional photolithography process to form a photoresist film 8 for the purpose of either simultaneously etching the metallic film 4 and the a-Si film 3, or etching the metallic electrode and then the a-Si film 3, the desired patterning being achieved on the metallic electrode 4 and the a-Si film 3, and the photoresist film 8 being then peeled away. By doing this, the desired shape is achieved for the metallic electrode 4 and the a-Si film 3.
Finally, an appropriate electrically conductive paste 5 is used to make a connection between the metallic electrode 4 of one solar cell element and the transparent conductive film 1 of another solar cell element, thereby forming the solar cell.
In either of the above-noted past examples, solar cell elements outside the range of the drawings have the same type of configuration, a plurality of solar cell elements formed on the substrate 1 being connected in series, so that a combined electromotive force is generated. After the above steps are performed, the solar cell is completed by applying a protective film over the upper surface.
In a method for manufacturing a solar cell in the past, however, etching is performed either two or three times, this being the etching of the transparent conductive film 2 of FIG. 2(B), the etching of the a-Si film 3 of FIG. 2(D), and the etching of the metallic electrode 4 of FIG. 2(F).
Because before performing each of the etching steps, a photoresist film is formed, photolithography is performed either two or three times, and three types of masks, having different patterns, are required to perform these photolithography steps.
Additionally, while a film is not contaminated by the etching step alone after it is formed, when the steps of patterning or etching and resist peeling are performed, there is an unavoidable film surface contamination. Additionally, even if sufficient cleaning is performed, it is impossible to avoid residues of contamination and water on the surface.
In a solar cell, because the boundary between the transparent conductive film and an a-Si film and the boundary between the a-Si film and the metallic electrode form paths for the movement of electrons and holes, contamination of these boundaries greatly affect the characteristics of the solar cell.
Therefore, if a single photolithography step were sufficient, in addition to a shortening of the manufacturing process, it would be possible to suffice with only one type of mask, thereby achieving a great reduction in manufacturing cost.
Additionally, if it is possible after forming the transparent conductive film, the a-Si film and the metallic electrode film over the entire surface to perform patterning and achieve a series connection of cells, it is possible to precisely control the boundaries between films, enabling the manufacture of a high-quality solar cell. Because film formation is performed by a single continuous step, there is a great shortening of the manufacturing process.
Accordingly, it is an object of the present invention to provide an improvement over the drawbacks of the method for manufacturing a solar cell of the past, by providing a high-quality solar cell made by a simplified manufacturing method, and a method for manufacturing such a high-quality solar cell.